Electronic component and method of manufacturing the same

ABSTRACT

An external terminal of an electronic component is provided with a lead base material and a metal thin film coating a surface of the lead base material, and an average value of a crystal size index is not less than 7, which is defined based on dimensions of a crystal particle in a direction perpendicular to the lead base material surface and in a direction parallel thereto, taken on a cut surface of the metal thin film defined by a given plane cutting the metal thin film in a direction perpendicular to the lead base material surface. Such constitution provides an electronic component having an external terminal coated with a metal thin film of a simple structure constituted of Sn or a Sn-based and substantially Pb-free alloy, formed by plating on a surface of a lead base material.

This application is based on Japanese patent application.No.2003-319789, the content of which is incorporated hereinto byreference.

BACKGROUND OF THE INVENION

1. Field of the Invention

The present invention relates to an electronic component, and morespecifically to an electronic component provided with an externalterminal including a lead base material constituted of a predeterminedmetal material, and a metal thin film coating a surface of the lead basematerial and including at least a first layer constituted of a materialsubstantially Pb-free and predominantly composed of tin, and to a methodof manufacturing such electronic component.

2. Description of the Related Art

When implementing an electronic component such as an integrated circuit(hereinafter abbreviated as “IC”), a transistor, a resistance, acondenser and so forth on a circuit board or the like, an externalterminal of the electronic component is electrically connected to aconductive electrode of the circuit board, via a low-melting solder. Inthis process, the external terminal of the electronic component has tohave sufficient solder wettability, so as to secure desired connectionreliability between the electronic component and the circuit board. Forthis purpose, a surface of the lead base material of the externalterminal is provided with a metal thin film constituted of Sn or anSn-based alloy mainly composed of Sn, formed in advance by a surfacefinishing process such as electrolytic plating.

Referring to the low-melting solder, an Sn—Pb alloy mainly containingtin (hereinafter designated by “Sn”) and an additive of lead(hereinafter designated by “Pb”) has been popularly used. Here, Sn isthe main ingredient of the alloy and serves as an adhesive. Also Pbconstitutes, together with Sn, a group of metals that can form alow-melting alloy, and serves to lower a melting point of the alloy andto enhance an adhesion strength thereof. Accordingly, an Sn—Pb alloy hasbeen widely utilized in implementing an electronic component on acircuit board, because of the advantage that a melting point can beeasily controlled by adjusting a ratio of those two ingredients, andthat these metals are inexpensive.

Referring also to the metal thin film to be formed on a surface of alead base material of an external terminal included in an electroniccomponent, the metal thin film formed by plating an Sn—Pb alloy has beenpopularly employed. It is because of the excellent wettability with alow-melting Sn—Pb solder as well as of the inexpensive cost that anSn—Pb alloy has been preferably employed for plating to form a metalthin film on a surface of a lead base material for an external terminal.

However, the Pb ingredient in the Sn—Pb alloy is harmful against a humanbody, and besides discarding a used electronic device incursenvironmental pollution. Accordingly, Pb is not desirable from anenvironmental viewpoint. Lately, therefore, a so-called Sn-based andsubstantially Pb-free alloy, which excludes Pb as an ingredient forconstituting a low-melting solder, has come to be popularly used whenimplementing an electronic component on a circuit board. Incorrespondence with such trend, on the part of the lead base material ofthe electronic component also, a Sn-based and substantially Pb (lead)free alloy has come to be widely used for plating a surface thereof, toform a metal thin film.

Here, “substantially Pb-free” means a concentration of Pb of less than 1wt %, preferably 0.1 wt %. On the other hand, “Pb contained material”contains Pb of 5 to 10 wt %.

When plating a Sn-based and substantially Pb-free alloy on a surface ofa lead base material to form a metal thin film, the key issue ismaintaining a desired low-melting solder wettabillty and securingdesired connection reliability, whichever metal may be adopted as anadditive to Sn.

However in case where a Sn-based and substantially Pb-free alloy isplated on a surface of a lead base material to form a metal thin film, afine metal whisker are prone to appear on a surface of an externalterminal under a circumstance of practical use of the electroniccomponent, unlike a case of forming a metal thin film by an Sn—Pb alloy.Such whisker may cause a short circuit between the external terminals,and the tendency becomes higher especially in an electronic componentsuch as an IC in which a multitude of external terminals are led out atfine intervals from a peripheral portion of the package body.Accordingly, restraining emergence of a whisker constitutes a criticalissue when forming an outer coating of a metal thin film by plating Snor a Sn-based and substantially Pb-free alloy on a surface of a leadbase material of external terminals disposed in an electronic component.

Referring to this issue, models of whisker emergence mechanism, as wellas methods of restraining whisker emergence for the respective modelsthus far proposed will be described below.

For example, JP-A Laid Open No.2002-246208 refers to a whisker thatemerges on an external terminal of a variable resistor. Morespecifically, the external terminal is provided with a first platedlayer consisting of copper (hereinafter designated by “Cu”) formed on asurface thereof, and a second plated layer consisting of Sn of 1 to 4 μmin particle diameter, formed on the first plated layer. The citeddocument states that a whisker emerges after long hours of use of suchvariable resistor, thereby causing the variable resistor to render anunstable output. FIG. 10 is a schematic cross-sectional view showing astate that a whisker has merged on an external terminal of the variableresistor. According to the JP-A Laid Open No.2002-246208, the externalterminal 113 is provided with a metal plate portion 109, the firstplated layer 110 consisting of Cu formed thereon, and also the secondplated layer 111 consisting of Sn formed on the first plated layer 110.Because of such structure, an oxide layer emerges on a surface of thesecond plated layer 111 after using the variable resistor for a longtime, thereby causing expansion of each particle constituting the secondplated layer 111, resulting in emergence of whisker 114 on the externalterminal 113, as shown in FIG. 10. And in order to prevent the whiskerfrom emerging in this way on the second plated layer, the JP-A Laid OpenNo.2002-246208 proposes fusing the plated particles of the second platedlayer to transform the particles into a uniform layer, after forming thefirst plated layer consisting of Cu and the second plated layerconsisting of Sn on the external terminal surface. The document statesthat this process can prevent expansion in volume of the platedparticles due to formation of an oxide layer despite a long term use ofthe variable resistor, and can resultantly restrain the whiskeremergence on the second plated layer consisting of Sn formed on theexternal terminal surface.

Also, JP-A Laid Open No.2001-110666 discloses providing an Sn-platedlayer consisting of fine particles not larger than 1 μm in averagecrystal particle diameter, on an outermost layer of an externalelectrode consisting of a plurality of layers, formed on a base materialsurface of an electronic component. More specifically, the externalelectrode includes a nickel (hereinafter designated by “Ni”) platedlayer formed on a thick film electrode located in contact with both endportions of a ceramic base material, and an Sn-plated layer being formedon the Ni-plated layer and constituting an outermost layer of theexternal electrode. The Sn-plated layer is formed by electrolyticplating of Sn, such that an average crystal particle diameter becomes 1μm or less. Such fine structure can restrain whisker emergence on theSn-plated layer, even under a circumstance of a temperature cyclealternately repeating a high temperature and a low temperature.

Also, Ryusuke Kawanaka et al., “Role of Lead in growth suppression andgrowth mechanism of Tin-proper whisker”, Japanese Association forCrystal Growth, Vol. 10, No.2, pp. 148-156, Dec. 15, 1983 reportsdifferent emergence mechanisms of a whisker.

FIG. 11 is a schematic drawing for explaining a whisker emergencemechanism stated in Ryusuke Kawanaka et al., “Role of Lead in growthsuppression and growth mechanism of Tin-proper whisker”. JapaneseCrystal Growth Society. Vol. 10, No.2, pp. 148-156, Dec. 15, 1983.According to this literature, an oxide layer grows with the lapse oftime on a surface of an Sn or an Sn-alloy plated layer formed on a metalsurface. Since such oxide layer often has an uneven thickness, aninternal stress produced inside the plated layer concentrates in adefective portion of the oxide layer where the layer is thin, andsqueezes out inner atoms, thereby causing a whisker to grow.

However, the method of fusing the plated particles in the Sn-platedlayer according to the JP-A Laid Open No.2002-246208 includes, forexample, placing internal and external terminals in a first furnaceprovided with an FIR heater for approx. 30 seconds for preheating up to220 degree centigrade, and passing them through a second heating furnaceprovided with a burner in approx. one second, for heating up to approx.900 degree centigrade. Treating at such a high temperature asemiconductor component provided with a resin material, which is lessresistant against heat than a metal, at a position close to a terminalincurs heat degeneration of the resin component (melting, carbonization,degradation, oxidation and so on), deterioration of materials of eachconfiguration, mechanical damage by vaporizing expansion of water in thepackage, irregular growth of the interfacial metal alloy layer to causedegradation in reliability of performance of the component.

Also, the JP-A Laid Open No.2001-110666 proposes forming an Sn-platedlayer consisting of fine crystal particles of not larger than 1 μm inaverage diameter, to thereby restrain whisker emergence under atemperature cycle. However, some data have shown a contradictory resultthat a whisker is more prone to appear when a crystal particle diameterof the Sn-plated layer is smaller (For example, JP-A Laid OpenNo.1990-170996).

According to studies on the whisker emergence mechanism so far achieved,including the foregoing JP-A Laid Open No.2002-246208, JP-A Laid OpenNo.2001-110666. JP-A Laid Open No.1990-170996, and Ryusuke Kawanaka etal., “Role of Lead in growth suppression and growth mechanism ofTin-proper whisker”, Japanese Crystal Growth Society, Vol. 10, No.2, pp.148-156, Dec. 15, 1983, it is generally considered that formation of aninterface reaction layer and a surface oxide layer, internal compressionstress in a plated layer, recrystallization, defect displacement,unmatched thermal expansion coefficient between a base material and aplated layer, a compression stress generated by a mechanical processingand so forth are promoting factors for the whisker growth. FIG. 10 showsone of the reported models, where upon formation of a thin oxide layeron an Sn-plated surface, an internal stress produced within theSn-plated layer concentrates in a defective portion of the oxide layerand easily squeezes out inner atoms, thereby permitting a whisker toemerge. Various approaches have been made as stated in the JP-A LaidOpen No.2002-246208, JP-A Laid Open No.2001-110666, JP-A Laid OpenNo.1990-170996, and Ryusuke Kawanaka et al., “Role of Lead in growthsuppression and growth mechanism of Tin-proper whisker”, JapaneseCrystal Growth Society, Vol. 10, No.2, pp. 148-156, Dec. 15, 1983, andyet exact details of the whisker emergence mechanism have not beenclarified.

Nevertheless, from the viewpoint of environmental protection, it isnecessary to promote the use of a substantially Pb-free material forplating an outer surface of an external terminal of an electroniccomponent. For this purpose, it is essential to restrain the whiskeremergence under a circumstance of practical use, effectively and at alow cost.

SUMMARY OF THE INVENTION

As a result of intensive studies to solve the foregoing problem, theinventor has discovered a method of restraining whisker growth, based onnewly established findings as described below.

FIGS. 4A and 4B are a schematic cross-sectional drawing and aperspective drawing of a metal thin film 2 respectively, for describingan Sn atom transfer relative to the whisker growth based on theobservation by the inventor. Referring to FIG. 4B, an arrow 30 marked ona particle boundary 4 schematically-show the transfer of the Sn atom.Actually the atom transfer is also considered to simultaneously takeplace inside a crystal particle, however such transfer is not shownbecause an amount of Sn atom transfer inside the crystal particle isrelatively small, especially under a low temperature. Irregular patternsin each crystal illustrated in FIG. 4B stand for unevenness of a surfaceof the metal thin film 2. Also, since the particle boundary 4 is locatedunder such uneven surface, the particle boundary 4 cannot be identifiedthrough the surface of the metal thin film 2 as it is, unless thesurface is polished so as to attain a section parallel to a surface ofthe lead base material 1. Now description will be given hereunder,referring to FIGS. 4A and 4B wherever necessary.

Firstly, samples were prepared which included an external terminalconstituted of a lead base material made of a nickel alloy containing42% of iron (hereinafter designated by “Fe-42% Ni-alloy”) or of a metalmaterial mainly containing copper. In the sample an outer surface of thelead base materials was plated with Sn or a Sn-based and substantiallyPb-free alloy under different plating conditions, so as to form a metalthin film. These samples were subjected to a temperature cycle test anda high temperature and high humidity test etc. to examine whether or nota whisker would emerge on the metal thin film. Thereafter, the sampleson which a whisker 20 has emerged on the metal thin film have beenpicked up, for close observation of the whisker 20 through a secondaryelectron microscope. As a result, it has been proven that a patternappropriate to a coating deposited by plating remains at a tip portionof the whisker 20, and that besides an entirety of the whisker 20 isconstituted of a single crystal.

This leads to an assumption that the whisker 20 is not a crystal thathas newly emerged after forming the metal thin film 2, but a crystalwhich one of the crystal particles originally formed in the platingdeposition process where the metal thin film 2 has been formed has grownin a specific direction. Accordingly, it has been concluded that adiameter of the whisker 20 has a correlation with a size of a crystalparticle 3 that is considered to be the origin of the whisker growth,included in the metal thin film 2.

Also, through the observation of a process of the whisker growth, it hasbeen assumed that Sn atoms constituting the whisker have gathered froman extensive area in the metal thin film 2 to thereby build the whisker,since a void or a variation (reduction) in thickness of the metal thinfilm is scarcely seen in the proximity of the position where the whiskerhas grown, despite a large number of Sn atoms have obviouslytransferred.

Further, based on these findings, it has been concluded that theemergence and growth of the whisker 20 largely depends on a size of thecrystal particle 3 constituting the metal thin film 2. Morespecifically, with respect to an energy for the emergence and growth ofthe whisker, such assumption has been made that a greater driving energyis required in order that a thicker whisker emerges from a largercrystal particle, and that a thicker whisker grows shorter by an equalamount of driving energy or supply of Sn atoms.

Then the observation was carried out on the assumption that the whiskergrowth rate is determined by a supply amount of Sn atoms from asurrounding area, and that the transference of Sn atoms takes placeaccording to a diffusion mechanism of Sn. And based on a characteristicof a whisker that it may rather emerge more prominently under a lowtemperature, an intergranular diffusion, which predominantly takes placeunder a low temperature, has been focused on. As a result, an assumptionhas been reached that a metal thin film constituted of larger crystalparticles, and having a smaller crystal particle boundary in a unitvolume, provides a slower whisker growth rate.

Also, based on the foregoing assumption that a whisker is nothing butone of the crystal particles formed in advance during the platingdeposition process, which has grown in a specific direction, it has beenconcluded that a metal thin film of a amorphous structure, which doesnot include a crystal particle that can grow as a whisker, can naturallyinhibit the whisker emergence.

Further, the prepared samples were classified into two groups inconsideration of the foregoing conclusions. A first sample groupconsisted of a plurality of untested samples extracted out of the lotsincluding the samples on which a whisker has emerged, while a secondsample group consisted of a plurality of untested samples extracted outof the lots including the samples on which a whisker has not emerged. Asa result of close observation of a metal thin film structure of thesetwo groups, especially focusing on a crystal particle size, thefollowing facts have been proven.

(1) A whisker has not emerged on a sample having large crystal particlesin a plane parallel to a lead base material surface, on a metal thinfilm surface.

(2) A whisker has not emerged either on a sample having smaller crystalparticles in a plane parallel to a lead base material surface, on ametal thin film surface, provided that the metal thin film is thicker.

(3) Upon evaluating a size of crystal particles utilizing a crystal sizeindex, the definition of which will be subsequently described, anaverage crystal size index of the first sample group has proved to be 3to 6, while that of the second sample group has proved to be 8 to 15.

The present invention has been conceived based on the foregoingfindings, as described hereunder.

According to the present invention, there is provided an electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of the lead base material, wherein the metal thin filmincludes at least a first layer constituted of a material substantiallyPb-free and predominantly composed of tin, and an average value of acrystal size index is not less than 7, when the crystal size index isdefined as (a+b)/2, where a and b respectively represent dimensions inμm of a crystal particle constituting the first layer in a directionperpendicular to the lead base material surface and in a directionparallel thereto, taken on a cut surface of the first layer defined by agiven plane cutting the first layer in a direction perpendicular to thelead base material surface.

Also, according to the present invention there is provided an electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of the lead base material, wherein the metal thin filmincludes at least a first layer constituted of a material substantiallyPb-free and predominantly composed of tin, and an average value of Xh/Xvis not less than 4, where Xv and Xh respectively represent dimensions ofa crystal particle constituting the first layer in a directionperpendicular to the lead base material surface and in a directionparallel thereto, taken on a cut surface of the first layer defined by agiven plane cutting the first layer in a direction perpendicular to thelead base material surface.

Also, according to the present invention there is provided an electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of the lead base material, wherein the metal thin filmincludes at least a first layer constituted of a material substantiallyPb-free and predominantly composed of tin, and the first layer is formedby plating.

Also, according to the present invention there is provided an electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of the lead base material, wherein the metal thin filmincludes at least a first layer constituted of a material substantiallyPb-free and predominantly composed of tin, and the first layer has aamorphous structure.

Such constitution permits providing a sufficiently small crystalparticle boundary in a unit volume of the metal thin film, for exampleby increasing a diameter of the crystal particles constituting the metalthin film, when forming the metal thin film mainly containing Sn on asurface of a lead base material of an external terminal included in anelectronic component. Accordingly, transference of Sn atoms, which isthe cause of the whisker emergence and growth in the metal thin film, isrestrained. Besides, even though a whisker should emerge, a greateramount of Sn atoms per unit length would be necessary for the whiskergrowth, since the whisker has a large diameter. As a result, the whiskeremergence in the metal thin film is restrained, and even though thewhisker should emerge, the subsequent whisker growth is restrained sothat the whisker can only grow short.

The present invention prevents an accidental short circuit caused by thewhisker emergence and growth, thereby permitting high-densityimplementation of electronic components, further resulting in upgradedreliability of the implemented components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic drawings for explaining the crystal sizeindex defined in the present invention.

FIGS. 2A and 2B are schematic perspective and plan views, respectively,showing an electronic component according to a first embodiment of thepresent invention.

FIG. 3A is a schematic cross-sectional view taken along the line P—P′ ofFIG. 2B.

FIG. 3B is an enlarged fragmentary cross-sectional drawing of anexternal terminal of FIG. 3A.

FIGS. 4A and 4B are schematic drawings for explaining an Sn atomtransference with respect to a whisker growth.

FIG. 5 is a plating current profile according to the first embodiment ofthe present invention.

FIGS. 6A and 6B are related to a heat treatment after plating, FIG. 6Ashowing graphs of a relation between a duration of time and a crystalparticle diameter when an Sn-alloy thin film formed by plating issubjected to a temperature of 150 degree centigrade, and FIG. 6B showinga heat treatment profile for once heating a plated metal thin film toits melting point or higher, and slowly solidifying the metal thin film.

FIG. 7 is a graph showing a whisker emergence status during a hightemperature and high humidity test, with respect to an IC according tothe first embodiment of the present invention.

FIG. 8 is a graph showing a whisker emergence status during atemperature cycle test, with respect to an IC according to the firstembodiment of the present invention.

FIG. 9 is a plating current profile according to a second embodiment ofthe present invention.

FIG. 10 is a schematic cross-sectional drawing for explaining a whiskeremergence mechanism disclosed in JP-A No.2002-246208.

FIG. 11 is a schematic cross-sectional drawing for explaining anotherwhisker emergence mechanism.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to the accompanying drawings, embodiments of the presentinvention will be described hereunder.

Firstly, the crystal size index will be described as this is one of theprincipal features of the present invention. Generally, a particlediameter is used to define a size of a crystal particle. However, sincea crystal particle is of an irregular three-dimensional shape, it isdifficult to measure a size exactly, and besides the interpretation isambiguous. Accordingly, the inventor has established a definition of the“crystal size index” through an undermentioned process, in order toquantitatively define a size of a crystal particle constituting a metalthin film. FIGS. 1A to 1C are schematic drawings for explaining thecrystal size index, showing a lead base material provided with a platedmetal thin film on its surface.

The crystal size index is defined as a sum of (a+b) divided by 2, wherea and b respectively represent a dimension of a crystal particle in μmin a direction perpendicular to a surface of the lead base material 1(hereinafter simply referred to as “base material surface”) and indirections parallel to the base material surface, taken on a cut surfaceof the metal thin film 2 defined by a (given) plane 40 cutting the metalthin film 2 in a direction perpendicular to the base material surface.As shown in FIG. 1B, when a given cutting plane 40 perpendicular to thebase material surface is passing through, for example, a line A-B on acutting plane 42 parallel to the base material surface, a dimension ofeach crystal particle in a direction parallel to the base materialsurface can be designated as b1 through b5 in this drawing. As isapparent from FIG. 1B, in case where the cutting plane falls on aposition distant from a central portion of a crystal particle, adimension b may be taken as a smaller value than it actually is, as theexample of b5. Here, when a direction perpendicular to the base materialsurface is regarded as a thicknesswise direction of the metal thin film2, a dimension b in a direction parallel to the base material surface isto be measured in the proximity of a thicknesswise center line of themetal thin film 2. In case where the metal thin film 2 includes aplurality of layers stacked in its thicknesswise direction, a dimensionof crystal particles that are located along a thicknesswise center lineof the metal thin film 2 is to be measured, though it has been proventhrough experiments that in most cases columnar crystal particles arealigned forming a single layer as shown in FIG. 1C, when the crystalparticles constituting the metal thin film 2 are duly controlled to havea large diameter according to the present invention.

An average crystal size index is to be worked out by first observing andmeasuring a crystal size index of m pieces (m is an integer not fewerthan 2) of crystal particles continuously aligned on a given cutsurface, and then dividing a sum of the crystal size index values by m,in other words by calculating an average value of the crystal size indexof the crystal particles exposed on the cut-surface. Arithmetically, theaverage crystal size index is obtained by the following formula, when acrystal size index of an nth crystal particle (n is an integer thatsatisfies 1≦n≦m) is designated by (a_(n)+b_(n))/2:

${AverageCrystalParticleSizeIndex} = {\frac{1}{2m}{\sum\limits_{n = 1}^{m}\;{( {a_{n} + b_{n}} ).}}}$

Now an electronic component according to a first embodiment of thepresent invention will be described hereunder.

In this embodiment, a resin-sealed (plastic-packaged) integral circuit(referred to “IC”) is taken up as an example of the electronic componentwithout limiting thereto, and description will be made on an externalterminal of such IC. FIGS. 2A and 2B are schematic perspective and planviews, respectively showing the IC according to this embodiment. FIGS.3A and 3B serve for more detailed description of the IC, and FIG. 3A isa schematic cross-sectional view taken along the line P—P′ of FIG. 2B,while FIG. 3B is an enlarged fragmentary cross-sectional drawing of anexternal terminal of the IC.

Referring to FIGS. 2A, 2B and 3A, 3B, the IC 10 of this embodiment isprovided with a chip mounting pad 11, a semiconductor chip 12 bonded onthe chip mounting pad 11 by for example an Ag paste (not shown), a metalfine wire 14 made of an Au wire or the like connecting an electrode (notshown) on the semiconductor chip 12 and an inner lead 13, a sealingresin 15 such as an epoxy resin enclosing the semiconductor chip 12, thechip mounting pad 11, the inner lead 13 and the metal fine wire 14 andso on, and an external terminal 5. Also, the external terminal 5 isprovided with a metal thin film 2 including a predetermined first layer,formed on a surface of the lead base material 1 by a surface finishingprocess such as electrolytic plating. The external terminal 5 and thecorresponding inner lead 13 include the lead base material 1 of aone-piece structure, and a surface of the lead base material 1 whichcorresponds to a region of the inner lead 13 is optionally plated withAg or Au. Further, the first layer included in the metal thin film 2 ofthis embodiment is of a structure that achieves an average crystal sizeindex of not less than 7.

According to this embodiment, the external terminal 5 of the IC 10 isprovided with the metal thin film 2 which only includes the first layer,formed by plating on a surface of the lead base material 1 constitutedof for example an Fe-42% Ni-alloy. The metal thin film 2 of thisembodiment is constituted of an Sn—Bi alloy layer predominantlycontaining Sn with approx. 1.5 wt % of Bi as an additive metal andsubstantially Pb-free. Practically, an acidic plating solution of acomposition as shown in Table 1 was selected for use as a plating bath,among the commercially available acidic plating solutions, and platingwas performed with a plating current profile shown in FIG. 5, to therebyform the metal thin film 2 of approx. 12 μm in thickness, constituted ofan Sn—Bi alloy layer.

TABLE 1 Component Concentration Sn²⁺ 18 g/L Bi³⁺ 0.6 g/L Free acid 1.0NAdditive for medium-speed plating 22 mL/L

Employing a plating current profile as shown in FIG. 5 makes it possibleto form large crystal particles in the metal thin film 2 of the IC 10 asin this embodiment. That is, it is preferable to maintain a relativelylow plating current density, for example 1 A/dm² or lower for apredetermined period after initiating plating (here, about 2 min). Then,crystal nucleation may be restrained during the plating process. Afterduration of the predetermined period, a sufficient plating time shouldbe ensured under a certain current density. An average crystal sizeindex of the metal thin film 2, formed in this way according to thisembodiment, has proved to be 11.9.

Also, a greater whisker restraining effect can be achieved by furtherincreasing the crystal particle size through a subsequent heat treatmentof a metal thin film constituted of large crystal particles formedaccording to the first embodiment. Such heat treatment may be performedin two ways; heating at a temperature equal to or lower than a meltingpoint of the metal thin film for a long time so that the crystalparticles gradually grow, or once heating to the melting point or higherof the metal thin film and then slowly solidifying the metal thin film.

FIG. 6A is a graph showing a relation between a duration of time and acrystal particle diameter when an Sn-alloy thin film formed by platingis subjected to a temperature of 150 degree centigrade. In view of thegraph it is understood that the crystal particles gradually grow withthe lapse of time. However, such a method as growing the crystalparticles over a long time under a melting point or lower of the metalthin film requires at least several tens of hours to grow the crystalparticles up to a desired large diameter. Accordingly, From theviewpoint of productivity, it is both practical and preferable to onceheat up the metal thin film to its melting point or higher and to slowlysolidify the metal thin film, since this method provides a desiredeffect in a short time. FIG. 6B is a heat treatment profilecorresponding to a method of once heating a plated metal thin film toits melting point or higher, and slowly solidifying the metal thin film.In this embodiment, the samples were placed in a belt furnace providinga nitrogen atmosphere, where the temperature was raised to approx. 240degree centigrade, which is equal to or higher than the melting point ofthe metal thin film. Then the metal thin film was cooled at a coolingrate of approx. 0.65 degree centigrade per second, at least until thetemperature dropped to approx. 140 degree centigrade. Here, in casewhere the metal thin film is constituted of Sn or an Sn-based alloy,setting the cooling rate at 3 degree centigrade per second at maximum,in a temperature range of 235 to 210 degree centigrade after heating upthe metal thin film to its melting point or higher, permits restrainingdeposition of fine crystal particles. This process permits achieving abetter crystal particle growth in particle diameter within 10 minutes orless, than the process of growing the crystal particles over a long timeunder a melting point or lower of the metal thin film.

Now, a whisker emergence status on the IC 10 of this embodiment,observed during accelerated tests including a high temperature and highhumidity test and a temperature cycle test, will be described. FIG. 7 isa graph showing a whisker emergence status during a high temperature andhigh humidity test (test conditions: atmospheric temperature 85 degreecentigrade, relative humidity 85%) with respect to the IC 10 of thisembodiment. The horizontal axis of the graph stands for duration of timeof maintenance at the high temperature and the high humidity, and thevertical axis a whisker emergence index (the greater the index is, themore whiskers have emerged), thereby showing a whisker emergence amountwith the lapse of time. Also, FIG. 8 is a line graph showing a whiskeremergence status during a temperature cycle test (test conditions:repetition of 30 minutes under −30 degree centigrade and 30 minutesunder 125 degree centigrade), with respect to the IC 10 of thisembodiment. The horizontal axis stands for a number of cycles, and thevertical axis a whisker emergence index, thereby showing a whiskeremergence amount with the progress of temperature cycles. Also, fourtypes of samples each having a different average crystal size index, allof which are provided with a metal thin film formed by plating, weresubjected to the both accelerated tests for comparison purpose. In viewof FIGS. 7 and 8, it is apparent that in the both accelerated tests thegreater the crystal size index is, the more effectively the whiskeremergence is restrained. With respect to a sample provided with a metalthin film including crystal particles of an average crystal size indexof 7 or greater as the IC 10 of this embodiment, the whisker emergenceindex has been suppressed below 1, even after 1000 hours of the hightemperature and high humidity test, and 1000 cycles of the temperaturecycle test.

Further, this embodiment may be modified such that an average value ofXh/Xv (Xv and Xh are values in the same unit) becomes not less than 4,where Xv and Xh respectively represent dimensions of a crystal particleconstituting the first layer in a direction perpendicular to a surfaceof the lead base material 1 and in a direction parallel thereto, takenon a cut surface of the first layer defined by a given plane cutting thefirst layer in a direction perpendicular to the surface of the lead basematerial 1. Alternatively, the metal thin film 2 may be constituted suchthat a total extended length of the crystal particle boundary of thefirst layer does not exceed 300 μm per 1000 μm² of observation area, ona cut surface of the first layer parallel to a surface of the lead basematerial 1. In case where the first layer has a structure that satisfieseither of these conditions, the whisker emergence and growth on themetal thin film 2 can be similarly restrained to the first embodiment.

An electronic component according to a second embodiment of the presentinvention will now be described hereunder.

Here, a difference of the electronic component of this embodiment fromthat of the first embodiment only lies in the external terminal, morespecifically in a structure of the first layer in the metal thin film ofthe lead base material surface, and the remaining portions may be thesame as the first embodiment. Accordingly, the subsequent descriptiononly refers to a structure of the first layer in the metal thin film ofthis embodiment. The difference from the first embodiment is that thefirst layer in the metal thin film has a substantially amorphousstructure. Forming the first layer in an amorphous structure eliminatesthe continuous particle boundary, through which the Sn atoms canefficiently transfer. Therefore, even though a whisker emerges, thetransference of the Sn atoms necessary for growth of the whisker isrestrained, and resultantly the whisker growth is restrained.

A preferable method of forming an amorphous structure in the metal thinfilm by plating may include preparing a plating solution having arelatively high concentration, setting a sufficient separation between acathode and an electrode of an object to be plated, applying a highvoltage to significantly increase a deposition rate, and further turningthe current on and off according to a predetermined profile. FIG. 9 isan example of showing such profile. As shown therein, a current having ahigh density is supplied, and the current is switched on and off in ashort cycle such as every 0.5 second so that growth of each of crystalsis suppressed. Since such plating method inhibits the growth of thecrystal particles that would constitute the Sn-alloy layer, the Sn-alloylayer is formed in a substantially amorphous structure. Consequently,the Sn-alloy layer does not have a continuous particle boundary, or acrystal particle itself which would serve as a nuclear for a subsequentgrowth of a whisker. In addition, even in case of forming by plating ametal thin film having such amorphous structure, crystal particles willgradually be formed after plating. However in this case the metal thinfilm obtains a structure that can restrain a whisker emergence, unlike astructure formed by directly depositing the crystal particles.

Further, a metal thin film having an amorphous structure may also beformed in the following method, in addition to the plating method.Melted Sn or an Sn-alloy may be applied by thermal spraying to anexposed surface of the lead base material so that the Sn or Sn-alloy israpidly cooled. In this way the lead base material can be coated withthe Sn or Sn alloy thin film having an amorphous structure, therebyforming an external terminal provided with the first layer of anamorphous structure. When applying this method to the IC 10 of theforegoing embodiment, the process may preferably include mounting asemiconductor chip on a lead frame, performing a wire bonding, sealingwith a resin and applying a heat shielding cover on the resin-sealedportion. Then the thermal spraying may be carried out to apply melted Snor an Sn-alloy only to an exposed surface of the lead base material sothat the Sn or Sn-alloy is rapidly cooled. As a result, the Sn orSn-alloy thin film coating the lead base material surface obtains anamorphous structure, thereby forming an external terminal provided withthe first layer of an amorphous structure. Here also, recrystallizationgradually takes place in the first layer formed as above, because ofheat to be applied in a subsequent process. However, crystal particlesformed by such recrystallization have a different orientation anddistribution from those obtained by an ordinary plating method, and therecrystallization does not result in formation of a structure thatpermits emergence of a whisker.

In addition, referring to a thickness of the first layer, it has beenproven that a preferable thickness range is not less than 5 μm and notmore than 30 μm, more preferably not less than 10 μm and not more than25 μm.

It is to be understood that the present invention is not limited to theforegoing embodiments, but that various modifications may be made withinthe scope and spirit of the present invention. For example, while theabove embodiments refer to a metal thin film that only includes thefirst layer, an underlying plated layer such as a Cu-plated layer,Ni-plated layer, or a zinc (Zn) plated layer may be provided on asurface of the lead base material, so that the first layer may be formedthereon. Alternatively, another layer such as an Au-plated layer, whichrestrains emergence of a whisker, may further be formed on the firstlayer.

Also, the first layer may be constituted substantially of pure Sn, or anSn-based alloy predominantly composed of Sn and containing at least oneout of the group consisting of Bi, Ag, Cu, indium (In) and Zn as anadditive metal. In case of employing Bi as the additive metal, it ispreferable that the first layer includes an Sn—Bi alloy containing notless than 0.5 wt % but not more than 4 wt % of Bi, and that an averagecrystal size index of such alloy is in a range of not less than 7 butnot more than 20. Also, in case of employing Ag as the additive metal,it is preferable that the first layer includes an Sn—Ag alloy containingnot less than 0.5 wt % but not more than 6 wt % of Ag, and that anaverage crystal size index of such alloy is in a range of not less than7 but not more than 20. Further, in case of employing Cu as the additivemetal, it is preferable that the first layer includes an Sn—Cu alloycontaining not more than 3 wt % of Cu, and that an average crystal sizeindex of such alloy is in a range of not less than 10 but not more than30.

Furthermore, while an Fe-42% Ni-alloy is adopted as the material of thelead base material 1 in the foregoing embodiments, an Fe-based alloy,Cu, or a Cu-based alloy predominantly containing Cu may be employedinstead.

As described above, the present invention provides an electroniccomponent having an external terminal coated with a metal thin film of asimple structure constituted of Sn or a Sn-based and substantiallyPb-free alloy, formed by plating on a surface of a lead base material.Yet such electronic component can effectively restrain emergence of awhisker on the outer plated layer under a circumstance of practical use.

What is claimed is:
 1. An electronic component comprising an externalterminal including a lead base material constituted of a predeterminedmetal material, and a metal thin film coating a surface of said leadbase material and including at least a first layer constituted of amaterial substantially Pb-free and predominantly composed of tin,wherein an average value of a crystal size index is not less than 7 whensaid crystal size index is defined as (a+b)/2, where a and brespectively represent dimensions in μm of a crystal particleconstituting said first layer in a direction perpendicular to said leadbase material surface and in a direction parallel thereto, taken on acut surface of said first layer defined by a given plane cutting saidfirst layer in a direction perpendicular to said lead base materialsurface.
 2. The electronic component as recited in claim 1, wherein saidfirst layer is constituted substantially of pure tin, or a tin-basedalloy predominantly composed of tin and containing as an additive metalat least one out of the group consisting of bismuth, silver, copper,indium and zinc.
 3. An electronic component comprising an externalterminal including a lead base material constituted of a predeterminedmetal material, and a metal thin film coating a surface of said leadbase material and including at least a first layer constituted of amaterial substantially Pb-free and predominantly composed of tin,wherein an average value of Xh/Xv is not less than 4, where Xv and Xhrespectively represent dimensions of a crystal particle constitutingsaid first layer in a direction perpendicular to said lead base materialsurface and in a direction parallel thereto, taken on a cut surface ofsaid first layer defined by a given plane cutting said first layer in adirection perpendicular to said lead base material surface.
 4. Theelectronic component as recited in claim 3, wherein said first layer isconstituted substantially of pure tin, or a tin-based alloypredominantly composed of tin and containing as an additive metal atleast one out of the group consisting of bismuth, silver, copper, indiumand zinc.
 5. An electronic component comprising an external terminalincluding a lead base material constituted of a predetermined metalmaterial, and a metal thin film coating a surface of said lead basematerial and including at least a first layer constituted of a materialsubstantially Pb-free and predominantly composed of tin, wherein saidfirst layer is formed by plating, and a total extended length of acrystal particle boundary of said first layer is not more than 300 μmper 1000 μm² of observation area, on a cut surface parallel to a surfaceof said lead base material.
 6. The electronic component as recited inclaim 5, wherein said first layer is constituted substantially of puretin, or a tin-based alloy predominantly composed of tin and containingas an additive metal at least one out of the group consisting ofbismuth, silver, copper, indium and zinc.
 7. An electronic componentcomprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material and including at least afirst layer constituted of a material substantially Pb-free andpredominantly composed of tin, wherein said first layer in said metalthin film has an amorphous structure.
 8. The electronic component asrecited in claim 7, wherein said first layer is constitutedsubstantially of pure tin, or a tin-based alloy predominantly composedof tin and containing as an additive metal at least one out of the groupconsisting of bismuth, silver, copper, indium and zinc.
 9. Theelectronic component as recited in claim 1, wherein said first layer isconstituted substantially of pure tin, and an average crystal size indexof said first layer is in a range of 7 to
 20. 10. The electroniccomponent as recited in claim 1, wherein said first layer includes atin-bismuth alloy predominantly composed of tin and containing not lessthan 0.5 wt % but not more than 4 wt % of bismuth, and an averagecrystal size index of said first layer is in a range of 7 to
 20. 11. Theelectronic component as recited in claim 1, wherein said first layerincludes a tin-silver alloy containing not less than 0.5 wt % but notmore than 6 wt % of silver, and an average crystal size index of saidfirst layer is in a range of 7 to
 20. 12. The electronic component asrecited in claim 1, wherein said first layer includes a tin-copper alloycontaining not more than 3 wt % of copper, and an average crystal sizeindex of said first layer is in a range of 10 to
 30. 13. The electroniccomponent as recited in claim 1, wherein said metal material is either ametal predominantly composed of copper, or an iron-nickel alloy.
 14. Amethod of manufacturing an electronic component comprising an externalterminal, including a lead base material constituted of a predeterminedmetal material and a metal thin film coating a surface of said lead basematerial, said metal thin film including at least a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, said first layer being formed by an electrolyticplating process, wherein said electrolytic plating process for formingsaid first layer includes an initial plating step of supplying a currenthaving a current density not greater than 1 A/dm² during an initialstage of a predetermined duration of time.
 15. The method as recited inclaim 14, further comprising a heat treatment step of heating said firstlayer at a predetermined temperature for a predetermined time after saidplating process, wherein said predetermined temperature in said heattreatment step is not more than a melting point of said first layer. 16.The method as recited in claim 14, further comprising a heat treatmentstep of heating said first layer at a predetermined temperature for apredetermined time after said plating process, wherein said heattreatment step includes melting step of heating said first layer at atemperature equal to or higher than a melting point thereof for apredetermined time and gradual cooling step of gradually cooling saidfirst layer after said melting step, and wherein a cooling rate in saidgradual cooling step is not more than 3 degree centigrade per second.17. An electronic component comprising an external terminal including alead base material constituted of a predetermined metal material, and ametal thin film coating a surface of said lead base material andincluding a first layer constituted of a material substantially Pb-freeand predominantly composed of tin, wherein an average value of a crystalsize index is not less than 7 when said crystal size index is defined as(a+b)/2, where a and b respectively represent dimensions in μm of acrystal particle constituting said first layer in a directionperpendicular to said lead base material surface and in a directionparallel thereto, taken on a cut surface of said first layer defined bya given plane cutting said first layer in a direction perpendicular tosaid lead base material surface, and wherein said first layer iscomposed of a plurality of crystal particles arranged in a single layeron said surface of said lead base material.
 18. An electronic componentcomprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material and including a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, wherein an average value of Xh/Xv is not less than 4,where Xv and Xh respectively represent dimensions of a crystal particleconstituting said first layer in a direction perpendicular to said leadbase material surface and in a direction parallel thereto, taken on acut surface of said first layer defined by a given plane cutting saidfirst layer in a direction perpendicular to said lead base materialsurface, and wherein said first layer is composed of a plurality of thecrystal particle arranged in a single layer on said surface of said leadbase material.
 19. An electronic component comprising an externalterminal including a lead base material constituted of a predeterminedmetal material, and a metal thin film coating a surface of said leadbase material and including a first layer constituted of a materialsubstantially Pb-free and predominantly composed of tin, wherein saidfirst layer is formed by plating, and a total extended length of acrystal particle boundary of said first layer is not more than 300 μmper 1000 μm² of observation area, on a cut surface parallel to a surfaceof said lead base material, and wherein said first layer is composed ofa crystal particle arranged in a single layer on said surface of saidlead base material.
 20. A method of manufacturing an electroniccomponent comprising an external terminal, including a lead basematerial constituted of a predetermined metal material and a metal thinfilm coating a surface of said lead base material, said metal thin filmincluding a first layer constituted of a material substantially Pb-freeand predominantly composed of tin, said first layer being composed of aplurality of crystal particles arranged in a single layer on saidsurface of said lead base material, and said first layer being formed byan electrolytic plating process, wherein said electrolytic platingprocess for forming said first layer includes an initial plating step ofsupplying a current having a current density not greater than 1 A/dm²during an initial stage of a predetermined duration of time.
 21. Anelectronic component comprising an external terminal including a leadbase material constituted of a predetermined metal material, and a metalthin film coating a surface of said lead base material and including afirst layer constituted of a material substantially Pb-free andpredominantly composed of tin, wherein said lead base material comprisesa plated layer underlying said first layer on which said first layer isformed, and wherein an average value of a crystal size index is not lessthan 7 when said crystal size index is defined as (a+b)/2, where a and brespectively represent dimensions in μm of a crystal particleconstituting said first layer in a direction perpendicular to said leadbase material surface and in a direction parallel thereto, taken on acut surface of said first layer defined by a given plane cutting saidfirst layer in a direction perpendicular to said lead base materialsurface.
 22. An electronic component comprising an external terminalincluding a lead base material constituted of a predetermined metalmaterial, and a metal thin film coating a surface of said lead basematerial and including a first layer constituted of a materialsubstantially Pb-free and predominantly composed of tin, wherein saidlead base material comprises a plated layer underlying said first layeron which said first layer is formed, and wherein an average value ofXh/Xv is not less than 4, where Xv and Xh respectively representdimensions of a crystal particle constituting said first layer in adirection perpendicular to said lead base material surface and in adirection parallel thereto, taken on a cut surface of said first layerdefined by a given plane cutting said first layer in a directionperpendicular to said lead base material surface.
 23. An electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material and including a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, wherein said lead base material comprises a platedlayer underlying said first layer on which said first layer is formed,and wherein said first layer is formed by plating, and a total extendedlength of a crystal particle boundary of said first layer is not morethan 300 μm per 1000 μm² of observation area, on a cut surface parallelto a surface of said lead base material.
 24. An electronic componentcomprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material and including a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, wherein said lead base material comprises a platedlayer underlying said first layer on which said first layer is formed,and wherein said first layer in said metal thin film has an amorphousstructure.
 25. A method of manufacturing an electronic componentcomprising an external terminal, including a lead base materialconstituted of a predetermined metal material and a metal thin filmcoating a surface of said lead base material, said metal thin filmincluding a first layer constituted of a material substantially Pb-freeand predominantly composed of tin, said first layer being composed of aplurality of crystal particles arranged in a single layer on saidsurface of said lead base material, and said first layer being formed byan electrolytic plating process, wherein said lead base materialcomprises a plated layer underlying said first layer on which said firstlayer is formed, and wherein said electrolytic plating process forforming said first layer includes an initial plating step of supplying acurrent having a current density not greater than 1 A/dm² during aninitial stage of a predetermined duration of time.
 26. An electroniccomponent comprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material, said metal thin film beingcomposed only of a first layer constituted of a material substantiallyPb-free and predominantly composed of tin, wherein an average value of acrystal size index is not less than 7 when said crystal size index isdefined as (a+b)/2, where a and b respectively represent dimensions inμm of a crystal particle constituting said first layer in a directionperpendicular to said lead base material surface and in a directionparallel thereto, taken on a cut surface of said first layer defined bya given plane cutting said first layer in a direction perpendicular tosaid lead base material surface.
 27. An electronic component comprisingan external terminal including a lead base material constituted of apredetermined metal material, and a metal thin film coating a surface ofsaid lead base material, said metal thin film being composed only of afirst layer constituted of a material substantially Pb-free andpredominantly composed of tin, wherein an average value of Xh/Xv is notless than 4, where Xv and Xh respectively represent dimensions of acrystal particle constituting said first layer in a directionperpendicular to said lead base material surface and in a directionparallel thereto, taken on a cut surface of said first layer defined bya given plane cutting said first layer in a direction perpendicular tosaid lead base material surface.
 28. An electronic component comprisingan external terminal including a lead base material constituted of apredetermined metal material, and a metal thin film coating a surface ofsaid lead base material, said metal thin film being composed only of afirst layer constituted of a material substantially Pb-free andpredominantly composed of tin, wherein said first layer is formed byplating, and a total extended length of a crystal particle boundary ofsaid first layer is not more than 300 μm per 1000 μm² of observationarea, on a cut surface parallel to a surface of said lead base material.29. An electronic component comprising an external terminal including alead base material constituted of a predetermined metal material, and ametal thin film coating a surface of said lead base material, said metalthin film being composed only of a first layer constituted of a materialsubstantially Pb-free and predominantly composed of tin, wherein saidfirst layer in said metal thin film has an amorphous structure.
 30. Amethod of manufacturing an electronic component comprising an externalterminal, including a lead base material constituted of a predeterminedmetal material and a metal thin film coating a surface of said lead basematerial, said metal thin film being composed only of a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, said first layer being composed of a plurality ofcrystal particles arranged in a single layer on said surface of saidlead base material, and said first layer being formed by an electrolyticplating process, wherein said electrolytic plating process for formingsaid first layer includes an initial plating step of supplying a currenthaving a current density not greater than 1 A/dm² during an initialstage of a predetermined duration of time.
 31. An electronic componentcomprising an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcoating a surface of said lead base material and including a first layerconstituted of a material substantially Pb-free and predominantlycomposed of tin, wherein an average value of a crystal size index is notless than 7 when said crystal size index is defined as (a+b)/2, where aand b respectively represent dimensions in μm of a crystal particleconstituting said first layer in a direction perpendicular to said leadbase material surface and in a direction parallel thereto, taken on acut surface of said first layer defined by a given plane cutting saidfirst layer in a direction perpendicular to said lead base materialsurface, wherein said crystal particle is comprised of a columnarstructure, and wherein said first layer is composed of a plurality ofcolumnar crystal particles arranged in a single layer on said surface ofsaid lead base material.
 32. An electronic component comprising anexternal terminal including a lead base material constituted of apredetermined metal material, and a metal thin film coating a surface ofsaid lead base material and including a first layer constituted of amaterial substantially Pb-free and predominantly composed of tin,wherein an average value of Xh/Xv is not less than 4, where Xv and Xhrespectively represent dimensions of a crystal particle constitutingsaid first layer in a direction perpendicular to said lead base materialsurface and in a direction parallel thereto, taken on a cut surface ofsaid first layer defined by a given plane cutting said first layer in adirection perpendicular to said lead base material surface, wherein saidcrystal particle is comprised of a columnar structure, and wherein saidfirst layer is composed of a plurality of columnar crystal particlesarranged in a single layer on said surface of said lead base material.33. An electronic component comprising an external terminal including alead base material constituted of a predetermined metal material, and ametal thin film coating a surface of said lead base material andincluding a first layer constituted of a material substantially Pb-freeand predominantly composed of tin, wherein said first layer is formed byplating, and a total extended length of a crystal particle boundary ofsaid first layer is not more than 300 μm per 1000 μm² of observationarea, on a cut surface parallel to a surface of said lead base material,and wherein said first layer is composed of a plurality of columnarcrystal particles arranged in a single layer on said surface of saidlead base material.
 34. A method of manufacturing an electroniccomponent comprising an external terminal, including a lead basematerial constituted of a predetermined metal material and a metal thinfilm coating a surface of said lead base material, said metal thin filmincluding a first layer constituted of a material substantially Pb-freeand predominantly composed of tin, said first layer being composed of aplurality of columnar crystal particles arranged in a single layer onsaid surface of said lead base material, and said first layer beingformed by an electrolytic plating process, wherein said electrolyticplating process for forming said first layer includes an initial platingstep of supplying a current having a current density not greater than 1A/dm² during an initial stage of a predetermined duration of time. 35.An electronic component, comprising: an external terminal including alead base material constituted of a predetermined metal material, and ametal thin film covering a surface of said lead base material, whereinsaid metal thin film has a first layer made of a material substantiallyPb-free; wherein said first layer is made of a material predominantlycomposed of tin; wherein an average value of a crystal size index of acrystal particle constituting said first layer is greater than or equalto 7 μm, wherein said crystal size index defines a value obtained bydividing a sum of a size in a first direction and a size in a seconddirection by 2, in said crystal particle seen in a cross section of saidfirst layer; wherein said cross section of said first layer is a givensurface cutting said first layer in said first direction; wherein saidfirst direction is a direction, which is perpendicular to said surfaceof said lead base material; and wherein said second direction is adirection, which is parallel to said surface of said lead base material.36. An electronic component, comprising: an external terminal includinga lead base material constituted of a predetermined metal material, anda metal thin film covering a surface of said lead base material, whereinsaid metal thin film has a first layer made of a material substantiallyPb-free; wherein said first layer is made of a material predominantlycomposed of tin; wherein an average value of Xh/Xv of a crystal particleconstituting said first layer is greater than or equal to 4, wherein Xvis a size in a first direction perpendicular to said surface of saidlead base material, in said crystal particle seen in a cross section ofsaid first layer; wherein Xh is a size in a second direction parallel tosaid surface of said lead base material, in said crystal particle seenin said cross section of said first layer; and wherein said crosssection of said first layer is a given surface cutting said first layerin said first direction.
 37. An electronic component, comprising: anexternal terminal including a lead base material constituted of apredetermined metal material, and a metal thin film covering a surfaceof said lead base material, wherein said metal thin film has a firstlayer made of a material substantially Pb-free; wherein said first layeris made of a material predominantly composed of tin; wherein said firstlayer is formed by plating; wherein a total extended length of a crystalparticle boundary of said first layer is less than or equal to 300 μmper 1000 μm² of observation area; and wherein said observation area is acut surface parallel to said surface of said lead base material.
 38. Anelectronic component, comprising: an external terminal including a leadbase material constituted of a predetermined metal material, and a metalthin film covering a surface of said lead base material, wherein saidmetal thin film has a first layer made of a material substantiallyPb-free; wherein said first layer is made of a material predominantlycomposed of tin; and wherein said first layer in said metal thin filmhas an amorphous structure.
 39. An electronic component, comprising: anexternal terminal including a lead base material constituted of apredetermined metal material, and a metal thin film covering a surfaceof said lead base material, wherein said metal thin film has a firstlayer made of a material substantially Pb-free; wherein said first layeris made of a material predominantly composed of tin; wherein an averagevalue of a crystal size index of a crystal particle constituting saidfirst layer is greater than or equal to 7 μm, wherein said crystal sizeindex defines a value obtained by dividing a sum of a size in a firstdirection and a size in a second direction by 2, in said crystalparticle seen in a cross section of said first layer; wherein said crosssection of said first layer is a given surface cutting said first layerin said first direction; wherein said first direction is a direction,which is perpendicular to said surface of said lead base material;wherein said second direction is a direction, which is parallel to saidsurface of said lead base material; and wherein said first layer iscomposed of a plurality of crystal particles arranged in a single layeron said surface of said lead base material.
 40. An electronic component,comprising: an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcovering a surface of said lead base material, wherein said metal thinfilm has a first layer made of a material substantially Pb-free; whereinsaid first layer is made of a material predominantly composed of tin;wherein an average value of Xh/Xv of a crystal particle constitutingsaid first layer is greater than or equal to 4, wherein Xv is a size ina first direction perpendicular to said surface of said lead basematerial, in said crystal particle seen in a cross section of said firstlayer; wherein Xh is a size in a second direction parallel to saidsurface of said lead base material, in said crystal particle seen insaid cross section of said first layer; wherein said cross section ofsaid first layer is a given surface cutting said first layer in saidfirst direction; and wherein said first layer is composed of a pluralityof crystal particles arranged in a single layer on said surface of saidlead base material.
 41. An electronic component, comprising: an externalterminal including a lead base material constituted of a predeterminedmetal material, and a metal thin film covering a surface of said leadbase material, wherein said metal thin film has a first layer made of amaterial substantially Pb-free; wherein said first layer is made of amaterial predominantly composed of tin; wherein said first layer isformed by plating; wherein a total extended length of a crystal particleboundary of said first layer is less than or equal to 300 μm per 1000μm² of observation area; wherein said observation area is a cut surfaceparallel to said surface of said lead base material; and wherein saidfirst layer is composed of a plurality of crystal particles arranged ina single layer on said surface of said lead base material.
 42. A methodof manufacturing an electronic component comprising an externalterminal, including a lead base material constituted of a predeterminedmetal material and a metal thin film covering a surface of said leadbase material, said metal thin film having a first layer made of amaterial substantially Pb-free and made of a material predominantlycomposed of tin, said first layer is composed of a plurality of crystalparticles arranged in a single layer on said surface of said lead basematerial, and said first layer being formed by an electrolytic platingprocess, wherein said electrolytic plating process for forming saidfirst layer includes an initial plating step of supplying a currenthaving a current density less than or equal to 1 A/dm² during an initialstage of a predetermined duration of time.
 43. An electronic component,comprising: an external terminal including a lead base materialconstituted of a predetermined metal material, and a metal thin filmcovering a surface of said lead base material, wherein said metal thinfilm has a first layer made of a material substantially Pb-free; whereinsaid first layer is made of a material predominantly composed of tin;wherein an average value of a crystal size index of a crystal particleconstituting said first layer is greater than or equal to 7 μm, whereinsaid crystal size index defines a value obtained by dividing a sum of asize in a first direction and a size in a second direction by 2, in saidcrystal particle seen in a cross section of said first layer; whereinsaid cross section of said first layer is a given surface cutting saidfirst layer in said first direction; wherein said first direction is adirection, which is perpendicular to said surface of said lead basematerial; wherein said second direction is a direction, which isparallel to said surface of said lead base material; wherein said firstlayer is composed of a plurality of crystal particles arranged in asingle layer on said surface of said lead base material; and whereinsaid first layer is the uppermost layer to be contacted with a solder.44. An electronic component, comprising: an external terminal includinga lead base material constituted of a predetermined metal material, anda metal thin film covering a surface of said lead base material, whereinsaid metal thin film has a first layer made of a material substantiallyPb-free; wherein said first layer is made of a material predominantlycomposed of tin; wherein an average value of Xh/Xv of a crystal particleconstituting said first layer is greater than or equal to 4, wherein Xvis a size in a first direction perpendicular to said surface of saidlead base material, in said crystal particle seen in a cross section ofsaid first layer; wherein Xh is a size in a second direction parallel tosaid surface of said lead base material, in said crystal particle seenin said cross section of said first layer; wherein said cross section ofsaid first layer is a given surface cutting said first layer in saidfirst direction; wherein said first layer is composed of a plurality ofcrystal particles arranged in a single layer on said surface of saidlead base material; and wherein said first layer is the uppermost layerto be contacted with a solder.
 45. An electronic component, comprising:an external terminal including a lead base material constituted of apredetermined metal material, and a metal thin film covering a surfaceof said lead base material, wherein said metal thin film has a firstlayer made of a material substantially Pb-free; wherein said first layeris made of a material predominantly composed of tin; wherein an averagevalue of a crystal size index of a crystal particle constituting saidfirst layer is greater than or equal to 7 μm, wherein said crystal sizeindex defines a value obtained by dividing a sum of a size in a firstdirection and a size in a second direction by 2, in said crystalparticle seen in a cross section of said first layer; wherein said crosssection of said first layer is a given surface cutting said first layerin said first direction; wherein said first direction is a direction,which is perpendicular to said surface of said lead base material;wherein said second direction is a direction, which is parallel to saidsurface of said lead base material; wherein said first layer is composedof a plurality of crystal particles arranged in a single layer on saidsurface of said lead base material; and wherein said average value ofsaid crystal size index is obtained by continuously arranged crystalparticles more than or equal to 5 particles.
 46. An electroniccomponent, comprising: an external terminal including a lead basematerial constituted of a predetermined metal material, and a metal thinfilm covering a surface of said lead base material, wherein said metalthin film has a first layer made of a material substantially Pb-free;wherein said first layer is made of a material predominantly composed oftin; wherein an average value of Xh/Xv of a crystal particleconstituting said first layer is greater than or equal to 4, wherein Xvis a size in a first direction perpendicular to said surface of saidlead base material, in said crystal particle seen in a cross section ofsaid first layer; wherein Xh is a size in a second direction parallel tosaid surface of said lead base material, in said crystal particle seenin said cross section of said first layer; wherein said cross section ofsaid first layer is a given surface cutting said first layer in saidfirst direction; wherein said first layer is composed of a plurality ofcrystal particles arranged in a single layer on said surface of saidlead base material; and wherein said average value of said crystal sizeindex is obtained by continuously arranged crystal particles more thanor equal to 5 particles.
 47. The electronic component as recited inclaim 17, wherein a thickness of said single layer is defined by athickness of each of said crystal particles.
 48. The electroniccomponent as recited in claim 18, wherein a thickness of said singlelayer is defined by a thickness of each of said crystal particles. 49.The electronic component as recited in claim 19, wherein a thickness ofsaid single layer is defined by a thickness of each of said crystalparticles.
 50. The method as recited in claim 20, wherein a thickness ofsaid single layer is defined by a thickness of each of said crystalparticles.
 51. The electronic component as recited in claim 31, whereina thickness of said single layer is defined by a thickness of each ofsaid crystal particles.
 52. The electronic component as recited in claim32, wherein a thickness of said single layer is defined by a thicknessof each of said crystal particles.
 53. The electronic component asrecited in claim 33, wherein a thickness of said single layer is definedby a thickness of each of said crystal particles.
 54. The method asrecited in claim 34, wherein a thickness of said single layer is definedby a thickness of each of said crystal particles.
 55. The electroniccomponent as recited in claim 39, wherein a thickness of said singlelayer is defined by a thickness of each of said crystal particles. 56.The electronic component as recited in claim 40, wherein a thickness ofsaid single layer is defined by a thickness of each of said crystalparticles.
 57. The electronic component as recited in claim 41, whereina thickness of said single layer is defined by a thickness of each ofsaid crystal particles.
 58. The method as recited in claim 42, wherein athickness of said single layer is defined by a thickness of each of saidcrystal particles.
 59. The electronic component as recited in claim 43,wherein a thickness of said single layer is defined by a thickness ofeach of said crystal particles.
 60. The electronic component as recitedin claim 44, wherein a thickness of said single layer is defined by athickness of each of said crystal particles.
 61. The electroniccomponent as recited in claim 45, wherein a thickness of said singlelayer is defined by a thickness of each of said crystal particles. 62.The electronic component as recited in claim 46, wherein a thickness ofsaid single layer is defined by a thickness of each of said crystalparticles.